In the field of electrochemistry, there is a well known electrochemical cell known as the chlor-alkali cell. In this cell an electric current is passed into a saturated brine (sodium chloride) solution to produce chlorine and caustic soda (sodium hydroxide). A large portion of the chlorine and caustic soda for the chemical and plastics industry is produced in chlor-alkali cells. Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane e.g., a hydraulically impermeable cation exchange membrane such as the commercially available NAFION manufactured by the E. I. duPont de Nemours and Co. Alternatively the separator can be a porous diaphragm, e.g, asbestos which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a precious metal coating to yield what is known in the art as a dimensionally stable anode. The cathodes employed in such chlor-alkali cells are generally steel. At these cathodes both caustic soda and hydrogen are produced. Chlorine is produced at the anode. This is a very energy intensive process.
A fairly recent development in the chlor-alkali cell technology is the development of various forms of oxygen (air) cathodes. Such cathodes can result in significant savings in the cost of electrical energy employed in the chlor-alkali cells. It is estimated that there is a theoretical saving of about 25% of the total electrical energy employed to operate chlor-alkali cells provided that the formation of hydrogen gas at the cathode can be prevented. Stated alternatively, about 25% of the electrical energy in a chlor-alkali cell is utilized in the formation of hydrogen at the cathode. The prevention of hydrogen formation, e.g., by reducing oxygen at the cathode resulting in the formation of hydroxyl ions, can lead to the aforementioned savings in the cost of electricity. This is one of the major purposes for oxygen (air) cathodes. However, such cathodes are in contact with the electrolyte caustic soda and are subjected to the wetting action thereof.
One known form of oxygen (air) cathode involves an active layer containing carbon particles, which may or may not be catalyzed using precious metal catalysts, such as, silver, platinum, etc. The pores of the active layer may become wetted with the caustic soda thereby significantly reducing its ability to provide oxygen to the catalytic sites resulting in a loss of activity of the air cathode.
In an attempt to overcome these difficulties, hydrophobic materials, e.g., polytetrafluoroethylene (PTFE) have been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the active layer, per se, or to a protective or backing sheet which is laminated or otherwise attached to the active layer. Some oxygen (air) cathodes have been disclosed as containing PTFE in both the active layer and in a backing sheet laminated thereto.